1. Field of the Invention
The present invention relates to an image reading apparatus of the type which uses an instant-on light source, particularly an instant-on fluorescent lamp, for irradiating each of successively transferred document sheets with light for reading the images of the document sheet.
2. Description of the Related Art
Typically, an image reading apparatus comprises a light source for irradiating a document sheet with light, and an image sensor for detecting the light reflected from the document sheet to generate image signals. An example of light source is an instant-on fluorescent lamp, whereas an example of image sensor is a CCD sensor.
For facilitating document feeding, an image reading apparatus is known which incorporates an automatic document feeder (ADF). In operation of an ADF type image reading apparatus, a plurality of document sheets are placed in a stack on a paper feed tray and successively transferred along a document transfer path. During transfer, an instant-on fluorescent lamp as a light source irradiates each transferred document sheet, and an image sensor detects the light reflected on the document sheet for generating electric image signals. In such successive document reading, normally, the fluorescent lamp is turned on at the time of feeding the first one of the plural document sheets and maintained in the ON state until the last document sheet is discharged.
On the other hand, the instant-on fluorescent lamp is known to exhibit luminous energy which progressively decreases with light-on time due to progressive temperature rise. Thus, if the lamp is kept on throughout the successive reading process, the reading quality deteriorates toward the end of the process particularly when performing color image reading. Further, since the lamp is continuously kept on, electric power is wastefully consumed in the reading interval between each two successive document sheets.